1. Field of the Invention
The present invention relates to a thermoelectric conversion device for conducting conversion between thermal energy and electric energy by the Peltier effect or the Seebeck effect.
2. Description of the Related Art
Thermoelectric power generation is a technology for directly converting thermal energy into electric energy with the use of the Seebeck effect, a phenomenon in which a temperature difference given to opposing ends of a substance causes a thermal electromotive force in proportion to the temperature difference, whereby electric power can be taken out by connecting a load externally and forming a closed circuit. This technology has been in practical use as power supplies for remote areas, power supplies for aerospace use, power supplies for military use, and so forth.
Thermoelectric cooling is a technology that utilizes the Peltier effect, a phenomenon in which heat is transferred by electrons carried by electric current. Specifically, when two substances having carriers with different signs, for example, a p-type semiconductor and an n-type semiconductor, are connected thermally in parallel and electrically in series, and a current is passed therethrough, heat absorption is caused at the junction of the two substances utilizing the difference in the directions of the heat flow, which reflects the difference in the signs of the carriers. This technology has been in practical use as local cooling devices such as for cooling electronic devices in a space station, wine coolers, and the like.
Generally, the performance of a thermoelectric conversion material is evaluated by a figure of merit Z, or a figure of merit ZT, which is made dimensionless by multiplying it by an absolute temperature T. The figure of merit ZT is an index represented as ZT=S2/ρκ with S, ρ, and κ of the substance, where S is Seebeck coefficient, ρ is electric resistivity, and κ is thermal conductivity. Conventional thermoelectric conversion materials have not reached to a practically usable level in terms of the evaluation based on the figure of merit ZT.
To date, many types of materials have been studied as thermoelectric conversion materials. For example, it has been reported that NaxCoO2, which is a layered oxide, shows good thermoelectric conversion performance (see JP 9-321346A and WO 03/085748). WO 03/085748 discloses, as a thermoelectric-conversion film, a NaxCoO2 film formed on the c plane of a sapphire substrate and having a c-axis orientation, i.e. having the c axis oriented perpendicularly to the substrate surface.
NaxCoO2 has a structure in which a CoO2 layer, which is an electrically conducting layer, and Na layer, which is an electrically insulating layer, are arranged alternately. As is clear from the foregoing equation, a lower electric resistivity is desirable to increase the figure of merit ZT. For this reason, attempts have conventionally been made mainly to exploit the thermoelectric conversion performance of the electrically conducting layer regarding its in-plane directions, when a layered oxide as represented by NaxCoO2 is used as the thermoelectric conversion material.
With a layered oxide, the electric resistance in the in-plane directions is reduced by attaining a good crystal orientation. For example, JP 2000-269560A discloses a sintered compact having a uniform crystal orientation. JP 2003-95741A also discloses a polycrystal substance in which crystals are orientated.
There have been proposed methods of manufacturing a substance having a uniform crystal orientation, which include: a method of manufacturing a substance in which crystals are oriented, using a plate-shaped template (see JP 2002-321922A and JP 2002-26407A); a method in which a sintered compact is pulverized and shaped, and thereafter heat-melted and cooled to cause crystallization (JP 2002-111077A); a method in which a source material is dissolved in a solvent and the resultant gel is baked to grow a plate-shaped crystal (JP 2003-34583A); and so forth.
All of these techniques are for reducing the electric resistivity in in-plane directions by improving the crystal orientation of layered oxides, and as a result, improving the thermoelectric conversion performance.
Nevertheless, the figures of merit ZT achieved by the above-mentioned methods remain only slightly above ZT>1, which is the level regarded as a guideline for practically usable substances, with the use of limited substances and in certain temperature ranges, and the current state is that the figures of merit fall far short of the level of ZT>3, which is thought as the level required for wider proliferation of thermoelectric conversion devices.
The following gives a summary of publications in which conventional thermoelectric conversion devices are disclosed: JP 9-321346A, JP 2000-269560A, JP 2003-95741A, JP 2002-321922A, JP 2002-26407A, JP 2002-111077A, JP 2003-34583A, JP 2003-133600A, JP 2002-270907A, JP 11-330569A (Paragraph [0002]), WO 03/085748, JP 2002-316898A, and JP 2002-141562A.